Learn Hardcore Swahili: Walimu wanatutazama wakati tunakimbia uwanjani. - Teachers are watching us while we run in the field.

wanatutazama = wa- (they; class 2 subject prefix) + -na- (present-tense/continuous marker “are …-ing”) + -tu- (us; 1st person pl. object prefix) + tazama (verb stem “watch”).
Literally: “they-are-us-watch” ⇒ “they are watching us.”

Why is the subject prefix in wanatutazama wa- and not something else?

Because walimu (“teachers”) is a class 2 noun (human plural). Verbs agree with their subject’s noun class, and class 2 uses wa-.

What does the -na- in wanatutazama and tunakimbia signify?

The infix -na- marks the present tense (often continuous).
In wanatutazama, wa- + -na- + … = “they are …,” and in tunakimbia, tu- + -na- + kimbia = “we are running.”

Why isn’t the pronoun sisi (“we”) written before tunakimbia?

Swahili normally indicates the subject with a prefix (tu- in tunakimbia). You may add sisi for emphasis (“sisi tunakimbia”), but it isn’t required.

What does wakati mean here, and how does it function?

wakati means “when” or “while.” It introduces the time clause “while we are running,” connecting it to the main action (“teachers are watching us”).

How is uwanjani formed, and why don’t we use a separate word for “in”?

uwanjani = uwanja (field; noun class 11) + -ni (locative suffix meaning “in/at”). Swahili typically expresses “in/on/at” by adding -ni rather than a separate preposition.

Can I switch the order of the clauses? For example, start with the time clause?

Yes. You can say:
Wakati tunakimbia uwanjani, walimu wanatutazama.
Both orders are grammatically correct.

How would you say “The teacher watches us while we run in the field” (singular teacher)?

Change walimu to mwalimu (class 1) and its verb prefix to a-:
Mwalimu anatutazama wakati tunakimbia uwanjani.

How would you say “They watch me as I run”?

Replace the object prefix -tu- (“us”) with -ni- (“me”) and adjust the subordinate clause subject:
Walimu wananitazama wakati ninakimbia.
Here wa- + na- + ni- + tazama = “they are watching me,” and ni- + na- + kimbia = “I am running.”